Carbonate Alkalinity

[waterbear]

This is a continuation (or side thread) of this thread
http://www.poolforum.com/pf2/showthr...9505#post29505
that I thought was better suited to the China Shop since it was starting to get a bit technical.

The only explanation I can come up with that is consistent with what these guys are saying is that injecting CO2 into the water increases the Carbonate Alkalinity, which is a true statement. However, there is a difference between carbonate alkalinity and Total Alkalinity. The former does not include [OH-] while the latter does. For the pool alkalinity tests, you measure Total Alkalinity, not Carbonate Alkalinity. For practical purposes, the two are very close in a pool (except at high pH), unless there is CYA present (in which case there is the CYA adjustment that can be made).

What can I say? Perhaps you can do an experiment if you've got one of these CO2 injectors. Just be sure to do the experiment in water that doesn't have other things going on (i.e. living things doing photosynthesis and respiration, etc.).

Anyway, I'm off on vacation. Talk to y'all after next week!

Richard

TA is usually defined as the amount of acid required to lower the pH of the sample to the point where all of the bicarbonate [HCO3-] and carbonate [CO3--] could be converted to carbonic acid [H2CO3]. This is the carbonic acid equivalence point or the carbonic acid endpoint. These equations show what happens to carbonate and bicarbonate as acid is added:

(1) H+ + CO3 ==> HCO3-

(2) H+ + HCO3- ==> H2CO3

For practical purposes and at the pH encountered in pools the contirbution to alkalinty of OH- is probably about .1% while the contirbution of bicarbonates is around 90%, carbonates around 7% and the remaining 3% would be composed of borates, phosphates, OH-, etc. This is, of course, with no cyanurates present. Cyanurates will contribute around 1/3 of the alkalinity at normal pool pH. Thus, for practical purposes TA will be essentially equal to carbonate alkalinity.

It would stand to reason (correct me if I am wrong) that if the amount of carbonic acid in the buffer system was reduced by forced gassing off CO2 this would result in the lowering of the TA since the reverse of the 2 reactions above would produce a lower level of both bicarbonate and carbonate.

Also, if we are truly interested in measuring TA and not carbonate alkalinity wouldn't it make sense to include any alkalinity contirbutred by cyanurates? The fact that we do correct for them indicates that we are interested in the carbonate alkalinity, since this is what is active in the formation of scale (calcium carbonate) in the water and is also what is used in calculating the LSI ( or any other SI applied to pools for that matter)